In 1972, enrollment of Medicare beneficiaries in health maintenance organizations (HMOs) was authorized, but participation remained limited and highly localized until recently. Since 1990, the number of Medicare-authorized HMOs has tripled, and enrollment in them has more than doubled. Despite these trends, evidence concerning the performance of Medicare HMOs, relative to conventional (fee-for-service) Medicare, is extremely limited, with results that are inconsistent, and possibly obsolete. The proposed study will employ a longitudinal observational design to compare the primary care performance and outcomes of conventional Medicare (CM) and managed Medicare (MM). Primary care performance refers to performance on each of 7 elements that define and distinguish primary care: accessibility, continuity, comprehensiveness, coordination, clinical management, a whole-person orientation, and a sustained clinician-patient partnership. Five outcomes will be monitored: changes in functional health, adherence to medical advice, HMO disenrollment, patient satisfaction, and mortality. The study population will include 15,000 Medicare beneficiaries drawn randomly from 59 large, mature HMOs, and from CM, in 14 states (MA, NY, PA, FL, IL, MN, TX, NM, AZ, CO, CA, OR, WA, HI). The Health Care Financing Administration will provide data on disenrollment, mortality, and plan characteristics. All other data will be obtained from patients at 3 intervals over a 24-month period using well-validated tools for primary care performance and outcomes assessment. Analyses will compare primary care performance and outcomes in CM and MM systems, and will examine differences within MM that are associated with HMO model-type, tax status (profit, non-profit), and other salient plan characteristics. Analyses will also examine the relationship between each primary care element and subsequent outcomes, to explore strong associations that have been observed cross-sectionally. Finally, the study will examine the concordance between measures of primary care performance and a measure of performance now mandated of all Medicare HMOs (HEDIS 3.0). The study will yield information unequaled in scope regarding the primary care and outcomes in conventional and managed Medicare. The study's findings will guide the formulation of meaningful and productive quality monitoring and improvement strategies for the Medicare program.